In OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) wireless communication networks, data streams are typically transmitted in parallel using multiple orthogonal sub-carriers or tones within a single channel. The use of orthogonal sub-carriers allows the sub-carriers' spectra to overlap, thus achieving high spectrum efficiency. An OFDM system maps coded or modulated information symbols, QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) symbols for instance, to sub-carriers in the frequency domain, and then generates a time domain signal for transmission using such a transformation technique as IFFT (Inverse Fast Fourier Transform). At a receiver, a time-to-frequency transformation, such as an FFT (Fast Fourier Transform), is used to convert a received time domain signal into the frequency domain. In order to recover transmitted source symbols correctly, the receiver aligns an FFT window with a corresponding IFFT window used at the transmitter and compensates for any frequency offset between the transmitter and the receiver.
Initial access to a communication network by a communication terminal involves a search operation to find available base stations and communication channels and a synchronization operation to synchronize the terminal to a base station. Dedicated physical channels, such as a downlink initial access channel (IACH) and a synchronization channel (SCH) for timing and frequency synchronization are used. A downlink IACH enables initial system access.
In wireless communication systems, the Base Transceiver Station (BTS) and User Equipment (UE) may have different transmission bandwidth capabilities. For example, a BTS and a UE may each have scalable bandwidths from 1.25 MHz to 20 MHz. However, in prior art systems, the bandwidth of the IACH is equal to the system transmission bandwidth and is thus variable depending on the system transmission bandwidth. This assumes that the UE reception capability is always equal to or larger than the transmission bandwidth. In prior art systems, a UE finds the system transmission bandwidth by changing the receive bandwidth and FFT size. This approach requires longer access time and complexity.